Those who practise the worship or cult of the devil, an aspect of heathen worship in its worst and most degraded form.
Among many barbarous peoples, offerings were made to evil and malignant gods, in order to placate them and avert their anger.
As each nation had its own gods, racial rivalry led them to consider the protecting divinities of an enemy as evil demons, hence those who worshipped what they considered good beings were devil-worshippers to other nations.
Such worship was not confined to heathens, for accounts of magical practises and occultation appear in the history of heretical sects in medieval Europe, and under the guise of various esoteric cults of today there are numerous devil-worshippers.
New Catholic Dictionary